1. Field of the Invention
The present invention relates to structure of power semiconductor devices.
2. Description of the Background Art
In a conventional power semiconductor device, power semiconductor elements are bonded on the top surface of an insulative substrate with solder, with the bottom surface of the insulative substrate bonded on the top surface of a metal base plate with solder. The power semiconductor elements are connected to electrodes on the insulative substrate through interconnection wires. The metal base plate is fixed on the top surface of a heat radiating fin with a plurality of bolts. Holes for insertion of the bolts are formed at a plurality of positions along the periphery of the metal base plate and at one position in the center of the metal base plate. A resin case is fixed on the top surface of the metal base plate, avoiding the one bolt disposed in the center of the metal base plate. The insulative substrate, to which the power semiconductor elements are bonded, is disposed within the case. The case is injected with gel to ensure insulation and to protect the wires. The case also contains epoxy resin placed on the gel in order to ensure airtightness (refer to Patent Document 1 shown below).
Patent Document 1: Japanese Patent Application Laid-Open No. 2000-228490 (FIGS. 1 and 2).
In such a conventional power semiconductor device, the case is fixed on the metal base plate, avoiding the bolt located in the center of the metal base plate. This causes a useless space in the center of the metal base plate, leading to an increase in size of the device.
Also, the case is expensive and increases the cost.
Furthermore, the gel injecting and curing processes and epoxy resin injecting and curing processes lower the productivity.
Moreover, when the power semiconductor device is used in an application where it is subjected to external vibrations (e.g. when used on a vehicle), the case and the metal base plate vibrate almost together but the gel vibrates after some delay, which causes relative dislocation between the gel and the case and metal base plate. Then the wires are pulled by the gel and may break due to fatigue at connections with the electrodes.